This invention relates to machines and processes for manufacturing concrete products, such as concrete pipe, manholes, catch basins, and the like, and more specifically, the invention relates to the “dry-cast” method of manufacturing concrete products which requires a system for providing vibration to the core and jacket of the concrete mold during the fill and pressure-head cycles of the manufacturing process.
In the dry-cast method of manufacturing, concrete vibration is necessary to consolidate and compact the concrete in the mold prior to curing. The inner mold, or core, of a concrete product mold therefore contains a vibration system which normally consists of eccentric weights mounted on a central shaft, or in some cases vibrators can be mounted on opposite sides of a rigid core for making rectangular shaped products. In any case, the vibrators are typically driven by electric or hydraulic motors. An example of a dry cast concrete pipe making machine using a central vibration system for a round concrete product is show in U.S. Pat. No. 4,708,621. As shown in this patent, the core is rigid and is used for producing a product of a particular size.
Manufacturers of concrete products typically produce products in different shapes and sizes which require the manufacturers to purchase and maintain jackets and rigid cores of different sizes. For rectangular shaped products, there are known and available adjustable forms which are panels secured together so that the form can be increased or decreased in size when desired by changing panels. However, when adjustable forms having removable panels are used, central core vibration systems have not been successfully used because the vibration will cause the panels that form the core to loosen and separate.
Therefore, when adjustable forms are used, the common and most economical vibration system that is currently used is a system in which small individual vibrators are mounted on both the jacket and the core. (FIG. 2 of the drawing shows an example of this on the jacket). When using this prior art method, the jacket and core form set must be built flexible enough to allow the vibrators to flex the structure and thereby transmit the vibration to the concrete. If the form set is built too rigid, it will not vibrate properly. The problem with building the form set flexible enough to vibrate the concrete, is that when concrete is placed in the form, the form begins to bow at the center of the sides. This is not desired as it leaves bows in the center of the product walls which create point contact at the center of the span when the product is installed in the field. It also leaves larger than desired gaps between the top and bottom joint rings and the product wall, which gap creates undesirable fins when the joint rings and form set is removed during the final steps of the concrete pipe manufacturing process.
Another problem with this known vibration system for adjustable forms is that it does not provide adequate vibration to properly compact the concrete. This inadequate vibration contributes to a condition that has been termed “slabbing off”.
There is therefore a need for an improved vibration system for adjustable cores that will provide advantages over the existing prior art systems.